1. Field of the Invention
The invention relates to a fuel injection control system of an internal combustion engine in which multiple fuel injections are performed.
2. Description of the Related Art
An internal combustion engine including fuel injection valves for supplying fuel directly into combustion chambers of cylinders is known in the art. When this type of engine having the direct injection type fuel injection valves operates at low temperatures, in particular, atomization of injected fuel is difficult to promote or proceeds at a reduced speed, and its diffusibility tends to deteriorate. In the internal combustion engine having the direct injection type fuel injection valves, therefore, the required amount of the fuel to be injected is not injected from each of the fuel injection valves at a time, but is injected over a plurality of injections (multiple injections) of smaller amounts of fuel at two or more different times on the intake stroke. In this case, the number of injections, injection times (points in time at which the respective injections are performed) and the amount of the fuel to be injected at each injection time, with regard to the multiple injections, are set based on operating conditions of the engine.
In Japanese Patent Application Publication No. 4-136451 (JP-A-4-136451), a fuel injection system is described in which a timer used for starting fuel injection is set at the time of a rise of a crank signal immediately after a cylinder discrimination signal is generated. A technology of determining a transient condition of the engine based on the rate of change of the engine speed is described in Japanese Patent Application Publication No. 2001-159365 (JP-A-2001-159365). With this technology, the time interval of reference position signals is increased when the rate of change of the engine speed is larger than 1, and the interval of reference position signals is reduced when the rate of change of the engine speed is smaller than 1. Japanese Patent Application Publication No. 9-158755 (JP-A-9-158755) describes a technology of accurately controlling the injection completion time with respect to the crank angle, by correcting the injection period in view of a change of the rotational speed of the engine when it changes from the time of generation of an injection command to the actual injection.
When multiple injections are performed, however, a rapid change in the rotational speed of the engine results in a large deviation of the actual fuel injection time of each of the multiple injections from the initially set fuel injection time (required injection time). Because of the deviation of the injection times, the actual amount of fuel deposited in the cylinder may differ from the assumed fuel deposition amount at the required injection time, or the amount of fuel returning into the intake system may vary, thus causing a difference between the amount of fuel actually used for combustion and the amount of fuel assumed, in a design stage, to be used for combustion. The difference in the amount of the fuel may also affect the air-fuel ratio of the resulting mixture, and, at worst, may result in misfiring. For these reasons, when the rotational speed of the engine changes rapidly, there arise possibilities of deteriorations in the driveability and/or exhaust emissions.